After lurking here for months (unfortunately only after I had applied to med school), I feel compelled to register to respond to this post. I have a BSN, so I know where you're coming from.
I would ask you to strongly reconsider the BSN to med school route. It is probably not at all true that your prereqs will be out of the way. Check your college catalog. I had to take many classes in addition to my degree program (additional higher algebra, trig, one more gen. chem, 2 org. chem, 2 physics, 2 biology--since nurses at my school just take 2 anatomy and physiology, plus microbiology, not the 2 semesters of pure bio) to get my med school prereqs. None of these classes counted for my degree, either, so it was a big gamble of time and money. I also took calculus and biochemistry on my own time/credit to enhance my application. This would all have been part and parcel, I believe, in a biology degree.
I was already an RN and decided to go back to school for a bachelor's because I was considering med school. I would have chosen biology had I known then what I know now! Besides the fact that you'll have to keep your med-school aspirations to yourself--nursing is a (rightly) proud profession and will try to steer you into the nurse practitioner route.
If this hasn't convinced you, then I would suggest a peek at the MSAR book. Very sobering statistics on the admission rates of applicants from other health professions. If I recall correctly, applicants with nursing degrees were the second-least accepted (seen as a career-hopper, etc., I guess, plus you're already in healthcare, as someone has noted above).
If your goal is strictly to get into medical school, I think the BSN route is a relatively poor choice for the reasons I have outlined above.
P.S. I did get into medical school despite all of the above, but it was a real pain to take all of those classes and take that kind of risk of time/money (but I managed a 35-37 MCAT when it was all over). Hope that helps.